Festo Belt Drives 2, Industrial Maintenance

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Job Sheet 3 – Variable-Speed Belt Drives
42
© Festo Didactic 54977-3C
To maintain belt alignment, the pulleys are arranged so that the movable flanges
are on the opposite sides of the belt. Any movement of the flange of the
adjustable pulley causes an opposite movement on the flange of the automatic
pulley.
Variable-speed belt drives with an intermediate variable-pitch pulley
When large distances and speed variations are required, using an intermediate
variable-pitch pulley is preferable, as shown in Figure 34. In such systems, a
fixed-pitch pulley is installed on both the driving and the driven shaft and a dual
variable-pitch pulley is used as an intermediate pulley. The movement of the
intermediate pulley increases the pitch on one side of the pulley, which
decreases the pitch on the other side. Moving the intermediate pulley toward the
driving shaft will result in a lower speed ratio and moving it away from the driving
shaft results in a higher speed ratio.
Figure 34. Variable-speed belt drive with an intermediate variable-pitch pulley.
Variable-pitch pulley alignment
Aligning variable-pitch pulleys is slightly different from aligning fixed-pitch pulleys.
The alignment depends on the width of the pulleys present in the system. If the
pulleys have the same width, the alignment procedures remain the same. In the
case of different widths, there are some specific requirements.
Since the belt must run in the center of each pulley, the alignment is ensured
when the straightedge is parallel to the belt, as Figure 35 shows. In other words,
the alignment of the belt relies on the resulting offset on each side of the pulley
having the smaller width. A ruler or a measuring tape can be used to make sure
the distance between the straightedge and all of the surface points of the
small-width pulley shown in Figure 35 are the same when the straightedge is laid
on each side of the large-width pulley.
Fixed-pitch pulley
Fixed-pitch pulley
Dual variable-pitch pulley